My take on this was the branch circuit ran only to that simplex receptacle for an air conditioner. See post #2. The other issue is the 115 volt a/c he is using trips the breaker on another receptacle that is on the correct voltage. He could also be talking about a tandem breaker, we are never going to know if he doesn't return to the thread so we can continue to sort this out.
If not then yes that's possible but he would have to check at the panel to be sure. If so then the 120 volt receptacle would make sense. However the a/c is a high wattage low resistance appliance and would likely have low voltage in the event of an open neutral on a multiwire, not in agreement with the stated symptoms.